Electrophotographic laser beam printers have prevailed as an image forming apparatus. The laser beam printer comprises a controller, an engine control unit which forms an image under the control of the controller, and a discharge option control unit which can switch a plurality of discharge bins. Some printers have a function of delivering printed paper sheets to different discharge bins while switching the discharge bins in forming (printing) an image.
Discharge operation of paper sheets to different discharge bins will be explained.
FIG. 6 shows a communication sequence when printing is continuously done on two paper sheets to different discharge bins. The reference numerals of the building components of a printer are those shown in FIG. 2.
A controller 201 transmits print reservation command 1 (601) and print reservation command 2 (602) for two paper sheets to an engine control unit 203, and discharge bin 1 discharge reservation 1 (603) and discharge bin 2 discharge reservation 2 (604) to a discharge option control unit 202. After transmitting discharge bin 2 discharge reservation 2 (604), the controller 201 acquires from the discharge option control unit 202 a time necessary to move from discharge bin 1 to discharge bin 2 (605).
The controller 201 transmits print start command 1 (606) to the engine control unit 203. The engine control unit 203 outputs /TOP signal 1 (607) for the first paper sheet and starts print operation.
In transmitting a print start command for the second paper sheet, the controller 201 must widen the interval between the first and second paper sheets by the time necessary to switch the discharge bin by the discharge option control unit 202.
At this time, if the controller 201 transmits a print start command before a normal print start timing, the engine control unit 203 ensures an optimal throughput and continues continuous printing (continues continuous printing without widening the interval between paper sheets). To prevent this, the controller 201 must transmit print start command 2 (609) at a timing when the interval between paper sheets enough to deliver transfer media to different discharge bins can be ensured.
FIG. 7 is a timing chart of the engine control unit when transfer media are delivered to different discharge bins. FIG. 7 assumes that print reservation commands for two paper sheets have already been transmitted from the controller 201.
If the controller 201 receives print start command 1 (704), the controller 201 starts a pre-rotation sequence. The engine control unit 203 applies a high charge AC voltage so as to rise at the end of the pre-rotation sequence (705). After the end of the pre-rotation sequence, the engine control unit 203 outputs /TOP signal 1 (714), and starts print operation on the first paper sheet.
To successively deliver paper sheets to different discharge bins, the controller 201 transmits print start command 2 (708) at a timing when the interval between paper sheets enough to deliver transfer media to different discharge bins can be ensured, i.e., a time C taken to switch the discharge bin after the normal print start timing (706).
The engine control unit 203 has not received any print start command till the normal (not switching the discharge bin) print start timing (706). Thus, after a post-rotation sequence is executed once, the engine control unit 203 waits for reception of print start command 2 (708), and then starts the pre-rotation sequence.
In the above sequence, the post-rotation sequence is executed after print operation on the first paper sheet. As a result, print operation on one paper sheet is repeated twice. The interval between paper sheets originally suffices to be widened by the time C taken to switch the discharge bin, but is widened by a time D further including the time of the pre-rotation sequence. A redundant down time is generated by the pre-rotation sequence for the second paper sheet.
To eliminate this down time, transmission of a print start command is waited without executing the post-rotation sequence even at the normal print start timing (706), and print operation starts simultaneously when a print start command is received.
FIG. 8 is a timing chart of the engine control unit 203 when the print start command is waited without executing the post-rotation sequence even if no print start command has been received until the normal print start timing but an unexecuted print reservation command has been received. FIG. 8 assumes that print reservation commands for two paper sheets have already been transmitted from the controller 201.
When the controller 201 receives print start command 1 (806), the controller 201 starts the pre-rotation sequence. The engine control unit 203 applies a high charge AC voltage so as to rise at the end of the pre-rotation sequence (807). Upon completion of the pre-rotation sequence, the engine control unit 203 outputs a /TOP signal (815), and starts print operation on the first paper sheet.
The controller 201 sends a print start command (809) the time C taken to switch the discharge bin after the next normal print start timing (816).
Although no print start command is transmitted till the next normal print start timing (807), the engine control unit 203 has already received a print reservation command for the second paper sheet, and waits for a print start command without starting the post-rotation sequence. Upon reception of the print start command (809), the engine control unit 203 outputs a /TOP signal (817), and starts print operation on the second paper sheet.
This sequence can prevent generation of a down time as shown in FIG. 7 because no pre-rotation sequence need be performed before printing on the second paper sheet even when the interval between paper sheets is widened.
In this case, an extra charge AC bias is applied by the discharge bin switching time C in comparison with normal continuous printing.
In general, the service life of a photosensitive drum depends on the rotation time of the photosensitive drum and the application time of a high charge AC voltage applied to the photosensitive drum. The service life of the photosensitive drum is often set in consideration of these factors.
For example, as for the high charge AC voltage, the application time is calculated on the basis of a high charge AC voltage applied for printing on one paper sheet (to be referred to as “intermittent printing” hereinafter).
FIG. 10 shows the application state of a high charge AC bias in intermittent printing. The high charge AC voltage is so applied as to rise immediately before an image formation start timing, and falls at the same time as the start of the post-rotation sequence (1004). The rise period A, the fall period B, and a period (between 1003 and 1004) during which the high charge AC voltage is applied during print operation are defined as a high charge AC voltage applied in intermittent printing, and the service life of the photosensitive drum is set.
In the sequence of FIG. 8, the application time of the high charge AC voltage becomes longer than an assumed application time of the high charge AC voltage, which is adopted for estimating the life time of the photosensitive drum, by a period E (=C−(A+B)). This means that the degradation rate of the photosensitive drum is faster than an assumed one.
FIG. 18 is a sequence chart relating to operation of the engine control unit 203. This is a sequence chart particularly for a case where the engine control unit 203 executes pre-processing (referred to below as a “pre-rotation sequence”), which is necessary in order to perform a printing operation, at the moment a print-reserve command is received from the controller 201.
First, when image information and a print instruction are accepted from the host computer 200, the controller 201 transmits a print-reserve command to the engine control unit 203 based upon the print instruction received (2410, 2411). Further, the controller 201 analyzes the received image information and converts it to bit data.
Upon receiving the print-reserve command, the engine control unit 203 starts the pre-rotation sequence (2411). The engine control unit 203 applies a high voltage such as an AC charging high voltage in such a manner that a high voltage will be obtained at the end of the pre-rotation sequence and also starts up an actuator required for the printing operation.
The controller 201 transmits a print-start command to the engine control unit 203 at the moment the analysis and conversion to bit data of the image information received from the host computer 200 are completed and it becomes possible to transmit a video signal to the engine control unit 203 (2412).
Following the end of the pre-rotation sequence, the engine control unit 203 waits for transmission of the print-start command from the controller 201, receives the print-start command and transmits the /TOP signal to start the printing operation (2412, 2420, 2421).
In a case where the engine control unit 203 has not received a print-reserve command and a print-start command by the next print-operation start timing (referred to below as “normal print-start timing”) for the purpose of continuing with successive printing, the engine control unit 203 suspends the printing operation and starts print-operation post-processing (referred to below as a “post-rotation sequence”) (2413). In the post-rotation sequence, the engine control unit 203 halts the application of all high voltages, inclusive of the AC charging high voltage, as well as actuator drive.
In accordance with the sequence described in connection with FIG. 18, print pre-processing by the controller 201 and the pre-rotation sequence performed by the engine control unit 203 can be executed in parallel and the printing operation can be started as soon as the print pre-processing by the controller 201 ends. As a result, the time required for the first printing operation can be shortened.
In this case, however, the AC charging high voltage is applied needlessly for a period of time equivalent to the difference (Tr−Te) between a time period Tr, which extends from the moment the controller 201 transmits the print-reserve command to the moment the controller 201 transmits the print-start command (namely the print pre-processing time of the controller 201), and a time period Te required for the pre-rotation sequence.
In general, the service life of a photosensitive drum depends upon the length of rotation time of the photosensitive drum and the length of time the AC charging high voltage is impressed upon the drum. In many cases, therefore, the lifetime of the photosensitive drum is set taking these factors into account. For example, with regard to the AC charging high voltage, the AC charging high voltage applied in a case where a single sheet is printed (referred to below as “intermittent printing”) is used as the reference when calculating the service life of the drum.
FIG. 17 illustrates application of a charging AC bias in intermittent printing. The AC charging high voltage is applied so as to rise immediately prior to the timing at which image formation starts, and decays at the same time that post-processing (the post-rotation sequence) for the printing operation starts (2304). The service lifetime of the photosensitive drum is set upon adopting rise time A of the AC charging high voltage, decay time B thereof and a period (2303 to 2304) in which voltage is applied during the print operation as the AC charging high voltage applied at the time of intermittent printing.
Accordingly, with the sequence of FIG. 18, the AC charging high voltage is applied for a length of time longer by (Tr−Te) than that set for application of the AC charging high voltage.
Thus, according to the prior art, the AC charging high voltage is applied for a period of time longer than that set in advance for application of the AC charging high voltage and, as a consequence, the photosensitive drum deteriorates faster than originally assumed.